Method for manufacturing a waveguide component with several layers stacked on a substrate and a waveguide component obtained using said method

ABSTRACT

A method for manufacturing an optical waveguide component with several layers stacked on a silicon substrate. The layers include a buffer layer, a first substrate, a guiding layer on the buffer layer and including an optical waveguide, and an outer layer as protection against external stresses. The layers are made from an organic-inorganic hybrid using a sol-gel process. An additional layer is disposed on the guiding layer so that a symmetrical structure is produced that has the same refractive index around the waveguide in the guiding layer.

[0001] The invention concerns a method for manufacturing a waveguide component with several layers stacked on a silicon substrate, of the type comprising a buffer layer arranged on the substrate, a guiding layer arranged on the buffer layer in which a waveguide device is fabricated, and an outer protection layer against external stresses. The invention also concerns a waveguide component obtained using this method.

[0002] Components of this type are already known in which the protection layer is directly deposited on the guiding layer.

[0003] The purpose of the present invention is to improve these known waveguide components, in particular to improve wave transmission properties and to allow the fabrication of components having greater integration density.

[0004] To achieve this purpose, the method of manufacture of the invention is characterized in that an additional layer is deposited on the waveguide layer so as to obtain a symmetrical structure having the same refractive index around the waveguide device made in the guiding layer.

[0005] According to one characteristic of the invention, the additional layer deposited on the guiding layer is a second buffer layer having the same refractive index as the first buffer layer.

[0006] According to another characteristic, the second buffer layer is made from the same solution as the first buffer layer deposited on the silicon substrate.

[0007] According to a further characteristic, the waveguide device is made in the guiding layer by exposing the zones in this layer, which are to form the waveguide device, to ultraviolet radiation through the additional layer deposited on the guiding layer.

[0008] According to yet a further characteristic, exposure to ultraviolet radiation is performed after fabricating the protection layer.

[0009] The waveguide component of the invention is characterized in that a second buffer layer is made on the waveguide layer, between the latter and the protection layer, so as to obtain a symmetrical structure having the same refractive index entirely around the waveguide device made in the guiding layer.

[0010] The invention will be better understood, and other purposes, characteristics, details and advantages thereof will become more clearly apparent in the explanatory description given below with reference to the appended, single, schematic FIGURE which is given solely as an example to illustrate an embodiment of the invention.

[0011] As illustrated in the single FIGURE, a waveguide component of the invention essentially comprises a silicon substrate 1 on which is stacked, successively from bottom to top, a first buffer layer 2, a guiding layer 3 intended to house a waveguide device 6, a second buffer layer 4 and an upper outer protective layer 5.

[0012] The function of the first buffer layer 2 is to insulate the waveguide device from the silicon substrate 1 whose very high refractive index relative to the index of the guiding layer would lead to high losses through radiating modes.

[0013] The material of this buffer layer, as of the other layers, is an organic-inorganic hybrid whose composition is chosen to have desired properties, in particular in respect of insulation, index difference and mechanical resistance. The technology used is the sol-gel process to carry out a solid deposit on a solid substrate such as silicon, using liquid phase precursors. According to this technology, the silicon substrate is immersed in the solution of liquid precursors. By withdrawing it at constant speed from the solution a liquid layer is obtained on the substrate. The liquid layer is then solidified by heating.

[0014] Under the invention, the solution from which the first buffer layer 2 is made contains two organic-inorganic precursors: 3-(Trimethoxysilyl)propyl methacrylate (MAPTMS) and the reaction product between zirconium n-propoxide and methacrylic acid (Zr(OPr)₄:MAA). Both are able to form both an organic network and a mineral network. For the buffer layer, only a mineral network is made by hydrolysis and polycondensation of the alcoxide groups.

[0015] Buffer layer 2 is dried at a temperature of between 90 and 120° C., advantageously at 100° C., for 15 minutes to one hour, advantageously for 30 minutes.

[0016] To fabricate guiding layer 3, the same solution is used as for the buffer layer, but 2.5% photoinitiator is added by weight of MAPTMSS+MAA. The guiding layer is deposited on the first buffer layer 2 using the same immersion-withdrawal technique as for this buffer layer.

[0017] The guiding layer is then dried at a temperature of between 60° C. and 80° C., advantageously at 60° C., for a time of 10 to 45 minutes, advantageously for 30 minutes.

[0018] The second buffer layer 4 is made from the same solution as the first buffer layer 2 and the guiding layer 3 without adding a photoinitiator. The layer is again deposited using the immersion-withdrawal technique on the guiding layer and is then dried at a temperature of between 60° C. and 80° C., advantageously at 60° C., for a time of 10 to. 45 minutes, advantageously for 30 minutes.

[0019] On this buffer layer 4, protection layer 5 is deposited, which is a hydrophobic and anti-scratch layer to protect the component against external stresses. It must be transparent to UVs. It is advantageous to choose an organomineral precursor for its fabrication which gives a strongly resistant layer that is highly inert to the environment, such as trimethoxysilane. The latter is simply hydrolysed with an aqueous 0.01N HCl solution. After the protection layer is deposited, it is dried at a temperature of 60° C. for 20 to 45 minutes, advantageously for 30 minutes.

[0020] After the layers are stacked, the stack is insulated through a mask placed on the protection layer by UV radiation, advantageously at a wavelength of 325 nm for a few seconds to a few minutes depending upon the photoinitiator used. Before this insulation operation the buffer layers and the guiding layer had the same refractive index. But during this insulation, in the irradiated zones of the guiding layer under the effect of radiation, an organic network is formed by polymerisation. When polymerised, the zones have a greater refractive index than the index of their surrounding material. The irradiated zones therefore form a waveguide device 6 in the guiding layer.

[0021] Subsequently, the stack is stabilised by heat treatment at a temperature of between 100° C. and 200° C. advantageously at 100° C., for a period of between 20 to 45 minutes, advantageously for 30 minutes. The mineral network is then sufficiently cross-linked to render inactive any further UV radiation, by sunlight for example.

[0022] Depending upon the composition chosen, the transverse size of the guide and the conditions of radiation exposure, i.e. time and intensity, it is possible to fabricate components suitable for wavelengths ranging from the visible up to 1.55 μm in particular.

[0023] In this embodiment, insulation is made after depositing all the layers. It may also be performed not with insulation in the form of UV radiation using a mask, but directly using a laser. It is through this UV insulation or Laser that polymerisation occurs in the guiding layer causing a change in refractive index allowing the creation of waveguides in the insulated parts.

[0024] According to another embodiment of the invention, the guiding layer is insulated immediately after it is deposited so as to make the insulated parts solvent-resistant. The non-insulated parts are then dissolved using an alcohol solvent, advantageously batanol 1, using the principle of UV photolithography. The second buffer layer is subsequently deposited followed by the protection layer.

[0025] Insulation could also be conducted after depositing the second buffer layer.

[0026] Insulation though the buffer layer makes it possible to avoid the etching operation and therefore simplifies the component manufacturing method. Omission of etching avoids a definition loss which would otherwise be caused. Since no etching is performed, the guides have no surface roughness, a factor which largely contributes to propagation losses though scattering.

[0027] It is to be noted that, with the method of the invention, it is very important that layers 3′, 4 and 5 only undergo partial drying. If this were not the case, any future insulation would not allow the fabrication of waveguides having satisfactory quality as the polymerisation of the organic network would be insufficient.

[0028] With the method just described, the component of the invention has a symmetrical structure so that the medium surrounding the waveguides made in the guiding layer has the same refractive index. By depositing a second buffer layer on the waveguide layer, which is therefore sandwiched between two identical insulating buffer layers, it is possible to fabricate components with several waveguide layers one on top of another, or other components of complex configuration having a high integration density. 

1. Method for manufacturing a waveguide component with several layers stacked on a silicon substrate, of the type comprising a buffer layer deposited on said substrate, a guiding layer deposited on the buffer layer and allowing the fabrication of a waveguide device, and an outer protection layer against external stresses, the layers being made in an organic-inorganic hybrid using the sol-gel process, characterized in that on guiding layer (3) an additional layer (4) is deposited so as to obtain a symmetrical structure having the same refractive index around the waveguide device made in the guiding layer (3).
 2. Method according to claim 1, characterized in that the additional layer (4) deposited on guiding layer (3) is a second buffer layer having the same refractive index as the first buffer layer (2).
 3. Method according to claim 2, characterized in that the second buffer layer (4) is made from the same solution as the first buffer layer (2) deposited on the silicon substrate (1).
 4. Method according to either of claims 2 or 3, characterized in that the waveguide device is made in guiding layer (3) by exposing the zones of this layer which are to form the waveguide device to ultraviolet radiation or to laser through the additional layer (4) deposited on guiding layer (3).
 5. Method according to claim 4, characterized in that exposure to ultraviolet radiation is made after fabricating the protection layer (5).
 6. Method according to any of claims 1 to 5, characterized in that the guiding layer (3) is made from the same solution as buffer layers (2) and (4) after adding a photoinitiator.
 7. Method according to any of claims 1 to 6, characterized in that the first buffer layer (2) on substrate (1) is dried after being deposited, at a temperature lying between 900 and 120° C., advantageously at 100° C., for a time of 15 minutes to one hour, advantageously for 30 minutes.
 8. Method according to any of claims 1 to 7, characterized in that the guiding layer (3) after being deposited is dried at a temperature lying between 60 and 80° C., advantageously at 60° C., for a time of 10 to 45 minutes, advantageously for 30 minutes.
 9. Method according to any of claims 1 to 8, characterized in that insulating layer (4) deposited on guiding layer (3) is dried after its deposition at a temperature lying between 60 and 80° C., advantageously at 60° C., for a time of 20 to 45 minutes, advantageously for 30 minutes.
 10. Method according to any of claims 1 to 9, characterized in that the protection layer, after being deposited, is dried at a temperature advantageously of 60° C. for a time of 20 to 45 minutes, advantageously for 30 minutes.
 11. Method according to any of claims 4 to 10, characterized in that ultraviolet radiation is used advantageously with a wavelength of 325 nm, radiation advantageously being applied for a time of a few seconds to a few minutes depending upon the photoinitiator used.
 12. Method according to any of claims 1 to 11, characterized in that the component after being insulated by ultraviolet radiation, is annealed at a temperature lying between 100 and 120° C., advantageously at 100° C., for a time of 20 to 45 minutes, advantageously for 30 minutes.
 13. Method according to any of claims 1 to 3, characterized in that the guiding layer (3) is insulated immediately after it has been deposited, by ultraviolet radiation or laser, in that the non-insulated parts are then dissolved, and in that the additional layer is deposited on the guiding layer after dissolution of the non-insulated parts.
 14. Waveguide component fabricated according to the method of any of claims 1 to 13, characterized in that an additional insulating layer (4) is made on the guiding layer, and in that the material around the waveguide device fabricated in guiding layer (3) has a symmetrical structure relative to the guiding layer having the same refractive index.
 15. Waveguide component according to claim 14, characterized in that the additional layer (4) deposited on guiding layer (3) is a second buffer layer made from the same solution as the buffer layer (2) deposited on the silicon substrate (1). 